Discourse on the regulation of Dioxins during the operation of RTO

Dioxins, typically white crystals, are a colorless, odorless, and extremely toxic fat-soluble substance. They consist of one oxygen atom linked to two chlorine-substituted benzene rings, forming polychlorinated dibenzofurans (PCDFs). Each benzene ring can be substituted with 1 to 4 chlorine atoms, giving rise to numerous isomers. Among them, PCDDs have 75 isomers and PCDFs have 135 isomers.

Dioxins are secondary pollutants produced during the incineration of VOCs. They possess varying degrees of toxicity, stability, wide distribution, and immense harm. Their toxicity is 130 times that of cyanide and 900 times that of arsenic, and they have irreversible "teratogenic, carcinogenic, and mutagenic" toxicity. They are regarded as "one of the most hazardous chemical substances in the world". Chlorine-containing organic exhaust gases will generate dioxins during the RTO incineration process. How to effectively control the production of dioxins is a key concern for RTO designers and business owners.

1. Conditions for the generation of dioxins
Benzene ring: A certain quantity of benzene rings is needed, whether in the synthesis reaction or in the heterogeneous reaction of precursors.
Chlorine source: Chlorine-containing substances are necessary to provide a certain amount of chlorine atoms during the formation process of dioxins.
Temperature: Temperature is one of the most influential factors affecting the formation of dioxins (300 - 500℃).
Metal catalyst: In the synthesis reaction and precursor catalytic reaction, even with sufficient carbon and nitrogen sources and a suitable temperature, without the existence of a catalyst, there will not be excessive generation of dioxins.
Oxygen: In experiments, it was observed that under hypoxic conditions, the concentration of dioxin production began to decline. The presence of oxygen is essential in the synthesis reaction, and as the oxygen concentration increases, the concentration of dioxin generation generally also rises.
Water: Water plays a certain role in the generation of dioxins. As an additional oxygen source, the presence of hydrogen reduces the oxidation degree of dioxins and provides hydroxyl radicals.
Reaction time: Numerous experiments have indicated that at an appropriate temperature, after 5 - 30 minutes, the generation rate of dioxins increases sharply and is completed within 2 - 4 hours.

High-temperature decomposition of dioxins
When the incineration temperature attains 750℃, dioxins commence to decompose and the C-Cl bond initially fractures. When the temperature reaches above 800℃, the C-O bond breaks. At this temperature, within a short period, the benzene ring substances in the above two decomposition products still exist, which is referred to as an unstable structure and still a prerequisite for the generation of dioxins. It is a reversible reaction process and can produce dioxins once again under appropriate temperature.
When the incineration temperature reaches above 850℃, the precursors of dioxins (benzene ring substances) can rapidly and thoroughly decompose into inorganic substances, resulting in the disappearance of the benzene ring organic compounds, which are the precursors of dioxins. Regardless of the temperature range, dioxin substances will no longer be generated, which is an irreversible reaction process.

Benzene compounds will disintegrate into benzene during high-temperature incineration. The above figure depicts the reaction process of benzene during high-temperature incineration. It can be observed from the figure that the benzene ring organic compounds vanish when the residence time exceeds 1.2 seconds. Currently, the common perception is that when the exhaust gas temperature reaches 850℃ and remains for 1.5 seconds, or when the temperature reaches 1050℃ and lasts for a few thousandths of a second, dioxins can be entirely decomposed into inorganic substances. The prerequisite substances for generating dioxins no longer exist, and dioxins are completely eradicated.

Explanation on reducing dioxin production during the RTO incineration process.

Residence time: Reasonably design the size of the combustion chamber.
Combustion temperature: Ensure that the combustion temperature of the exhaust gas in the combustion chamber is within the range of 850-950℃.
Eddy: The flue gas is fully mixed in the high-temperature area of the furnace, enabling it to combust thoroughly.
Shorten the rapid cooling time of the high-temperature exhaust gas in the regenerative chambers, ensuring that the residence time of the exhaust gas in the medium-temperature range (300℃～500℃) is less than 0.5s, thereby reducing the production of dioxins.
Thermal storage ceramics possess adsorption functions, and the dioxins adsorbed in the ceramics will re-enter the combustion chamber for high-temperature decomposition along with the purging air.